The present invention relates to switch mode power converters and, in particular, to the protection of such converters against overload and short circuit conditions.
The conventional method of protecting switch mode power converters against overload or short circuit conditions is to turn OFF the power switch of the converter when the power switch current exceeds a predetermined value. In theory, this approach limits the output current to a safe value for the converter. Furthermore, the control method based on sensing the power switch current provides a substantially fast response time.
However, due to practical constraints, sensing the power switch current alone may not be sufficient for overload protection. Assuming ideal conditions, the above described method will limit the peak current in the power switch to a constant value. Depending upon the converter's topology and component values, the ratio of the output current of the converter to the peak current of the power switch can increase significantly as short circuit condition approaches in the output circuit. Consequently, an output current runaway may result. Furthermore, the power switch cannot be turned OFF instantly when its current exceeds the predetermined threshold. The turn OFF delay will allow a current overshoot, exacerbating the output current runaway.
Prior methods for solving the problems described above are either very complex or have relatively poor performance. For example, one method described previously is the use of a shunt resistance in the output circuit for sensing output current. Although, providing a shunt resistance is relatively simple, it causes undesired power dissipation. Another method for overload protection is described in U.S. Pat. No. 3,768,012 issued to Wilkinson and is illustrated again in FIG. 1. As described in more detail hereafter, the method illustrated in FIG. 1 may not provide accurate current measurements and requires additional undesirable windings.
Hence, there is a need for an improved circuit that can exhibit substantially ideal current characteristics.